The first thing you will need to do is find a black and white photo. I
decided to try using the most famous black and white photo I know of,
"Migrant Mother" taken by Dorothea Lange in 1936. Open the image in The
Gimp.

The first thing I like to do with all my projects is make a copy of the
layer. This way if I mess up, I can always go back to the original.

I also like giving the layers appropriate names to make things less confusing in the future.

Next, you will want to right click on the image and create a layer mask.

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Set the background as white.

Go ahead and make a copy of this as well.

You will need to make sure that the image is not in Grayscale mode. Go to Image>Mode>RGB and set the mode to RGB.

Now go to colors and select either Color Balance, Hue-Saturation, or Colorize. I tend to prefer Colorize.

For the Colorize dialog to actually come up, we need to make sure our
image is selected and not the layer mask (look below). The image is on
the left and layer mask is on the right.
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With Color Balance or Colorize, we will now want to try to get the image
to be same color of the object we want to color. I started with the
collar. I made sure I created a copy of the image and layer mask I could
use specifically for the collar.

If you use Colorize, it will start out bluish. Adjust the hue until you find the color you like.

I eventually got it to a red color I liked.

After you have the color you want, you will now need to select the layer mask by clicking on it.

Click on the right rectangle. This is the layer mask.

Now go to Colors and Invert the colors. Once the colors are inverted, it will look like the original without any color.

Next I used the selection tool to select the collar (I don't suggest
doing this... it is much easier to just use the paintbrush and select
the white color and paint).

Once I selected the collar, I used the paint bucket tool to fill it as white. Again... painting is easier.

Once you have that layer how you want it, create a new copy and repeat the process.

For my next layer I did the skin on the mother. I used the paintbrush tool this time. Much easier.

Keep on repeating the process for every object that is a different color (can be quite tedious but it quite fun).

One thing I should mention; don't worry too much about getting the
colors correct. Close enough is good enough. Changing the colors later
is quite easy.

Here you can see my different layers.

It is hard to get the colors right because it changes the colors for
everything until you invert... don't worry; we will adjust later if you
don't like the colors.

If you decide you want to go back and change a color, just select the
layer and go back to colorize (make sure the layer mask is NOT
selected).

I didn't like the orange sweater I did, so I made it a more brownish
color. I did this using colorize and dropped the saturation down quite a
bit.

After I had done all the layers, I started going back and adjusted the
opacity on the layers to make it look a bit more natural and not over
saturated.